The present invention relates to a light emitting diode utilizable as a light source for a triangulation type distance measuring device such as is used in an automatic focus control camera.
A light emitting diode is known which includes a pair of semiconductor layers with a planar P-N junction therebetween, and which is provided with at least one surface formed by cutting the semiconductor layers across the P-N junction, the edge of the junction emitting light in a direction parallel with the P-N junction, such as is disclosed in Japanese Patent Application Laid Open No. 54-40662 and in copending U.S. patent application Ser. No. 940,135, filed Sept. 6, 1978, and assigned to the same assignee as the present application.
FIG. 1 is a schematic illustration of such a prior art light emitting diode. As shown in FIG. 1, the light emitting diode comprises a P-type semiconductor layer P.sub.1 and an N-type semiconductor layer N.sub.1 with a planar P-N junction 2 formed therebetween. The entire upper surface of the P-type semiconductor layer P.sub.1 is coated with AuZn deposited by an evaporation deposition technique to form an electrode A.sub.1 to which a wire W.sub.1 is bonded to supply power to the diode. The N-type semiconductor layer N.sub.1 has firmly mounted on the lower surface thereof an electrically conductive substrate (not shown), which serves as the counter electrode to the electrode A.sub.1.
Four side surfaces such as 4 and 6 are formed by cutting the semiconductor layers P.sub.1 and N.sub.1 across the P-N junction so that light beams a, b, c and d are emitted from the edges of the junction in a direction parallel with the plane of the junction 2. Any one of the light beams a, b, c and d can be utilized as a source of light to be projected toward an object in a triangulation type distance measuring device.
However, the above-described ordinary light emitting diode has an undesirable characteristic that the light emitting efficiency decreases when the current supply is increased for the purpose of increasing the absolute intensity of the emitted light due to an increase in the temperature of the P-N junction. In other words, a greater part of the increase in the current supplied does not effect a desirable increase in intensity, but rather is consumed by the increased temperature of the P-N junction.